1. Field of the Invention
The invention is directed to a longitudinal adjusting device for seats, especially motor vehicle seats, whose backrest which is adjustable in selectable inclination positions is swivelably connected with a seat part, and the seat part is fastened to running rails, wherein every running rail is displaceable longitudinally in a guide rail fixed to the vehicle floor and at least one running rail can be secured to the guide rail in selectable longitudinal positions by means of a locking device, and at least one running rail is supported at the guide rail via a spring member for purposes of a forward displacement of the running rails.
2. Description of the Related Art
Longitudinal adjusting devices for seats of the type mentioned above are known in many constructional forms as can be seen, for example, from DE 32 26 198 A1or U.S. Pat. No. 4,143,911. Regardless of whether the displacement of the seat relative to the floor of the vehicle serves only for adjusting the longitudinal position of the seat or, when installed in two-door passenger automobiles, if it also serves to provide more space for rear seat passengers to board the vehicle, a spring element--e.g., an inherently elastic tension member--which reinforces the forward displacing movement of the seat is, as a rule, constructed as a helical tension spring which is connected on one hand with the running rail carrying the seat and, on the other hand, with either an articulation point of the vehicle floor or the guide rail which is fixed to the floor. Of course, it is also known to use helical pressure springs, wherein the articulation points are then reversed in a corresponding manner. Since a relatively large spring path is required for the longitudinal displacement of the seat, the selection of the springs according to their spring characteristic proves difficult inasmuch as the effective force should change only insignificantly from the start to the end of the displacing movement of the seat. However, this conflicts with the characteristics of helical tension springs and helical pressure springs of the type mentioned above which start at zero at the beginning of the spring path and, as the spring path advances, increase steadily to a maximum value at the end of the spring path. In order to achieve usable force ratios, springs of this type must have many turns and therefore have considerable length. This in turn requires a large space requirement for accommodating such helical tension springs and helical pressure springs.